With the development of radio and space technologies, several satellite-based navigation systems have already been built and more will be in use in the near future. One example of such satellite-based navigation systems is Global Positioning System (GPS), which is built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random number (PRN) code phase. Each satellite transmits signals using a unique 1023-chip long PRN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal sequence. During the acquisition stage, the code phase search step is a half-chip for most navigational satellite signal receivers. Thus the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
Coherent integration and noncoherent integration are two commonly used integration methods to acquire GPS signals. Coherent integration provides better signal gain at the cost of larger computational load, for equal integration times.
The signals from the navigational satellites are modulated with navigational data at 50 bits/second. This data consists of ephemeris, almanac, time information, clock and other correction coefficients. This data stream is formatted as sub-frames, frames and super-frames. A sub-frame consists of 300 bits of data and is transmitted for 6 seconds. In this sub-frame a group of 30 bits forms a word with the last six bits being the parity check bits. As a result, a sub-frame consists of 10 words. A frame of data consists of five sub-frames transmitted over 30 seconds. A super-frame consists of 25 frames sequentially transmitted over 12.5 minutes.
The first word of a sub-frame is always the same and is known as TLM word and first eight bits of this TLM word are preamble bits used for frame synchronization. A Barker sequence is used as the preamble because of its excellent correlation properties. The other bits of this first word contains telemetry bits and is not used in the position computation. The second word of any frame is the HOW (Hand Over Word) word and consists of TOW (Time Of Week), sub-frame ID, synchronization flag and parity with the last two bits of parity always being ‘0’ s. These two ‘0’ s help in identifying the correct polarity of the navigation data bits. The words 3 to 10 of the first sub-frame contains clock correction coefficients and satellite quality indicators. The 3 to 10 words of the sub-frames 2 and 3 contain ephemeris. These ephemeris are used to precisely determine the position of the GPS satellites. These ephemeris are uploaded every two hours and are valid for four hours to six hours. The 3 to 10 words of the sub-frame 4 contain ionosphere and UTC time corrections and almanac of satellites 25 to 32. These almanacs are similar to the ephemeris but give a less accurate position of the satellites and are valid for six days. The 3 to 10 words of the sub-frame 5 contain only the almanacs of different satellites in different frames.
The super frame contains twenty five consecutive frames. While the contents of the sub-frames 1, 2 and 3 repeat in every frame of a superframe except the TOW and occasional change of ephemeris every two hours. Thus the ephemeris of a particular signal from a satellite contains only the ephemeris of that satellite repeating in every sub-frame. However, almanacs of different satellites are broadcast in-turn in different frames of the navigation data signal of a given satellite. Thus the 25 frames transmit the almanac of all the 24 satellites in the sub-frame 5. Any additional spare satellite almanac is included in the sub-frame 4.
The almanac and ephemeris are used in the computation of the position of the satellites at a given time. The almanacs are valid for a longer period of six days but provide a less accurate satellite position and Doppler compared to ephemeris. Therefore almanacs are not used when fast position fix is required. On the other hand, the accuracy of the computed receiver position depends upon the accuracy of the satellite positions which in-turn depends upon the age of the ephemeris. The use of current ephemeris results in better position estimation than one based on non-current or obsolete ephemeris. Therefore it is necessary to use current ephemeris to get a fast receiver position fix.
A GPS receiver may acquire the signals and estimate the position depending upon the already available information. In the ‘hot start’ mode the receiver has current ephemeris and the position and time are known. In another mode known as ‘warm start’ the receiver has non-current ephemeris but the initial position and time are known as accurately as in the case of previous ‘hot start’. In the third mode, known as ‘cold start’, the receiver has no knowledge of position, time or ephemeris. As expected the ‘hot start’ mode results in low Time-To-First-Fix (TTFF) while the ‘warm start’ mode which has non-current ephemeris may use that ephemeris or the almanac resulting in longer TTFF due to the less accurate Doppler estimation. The ‘cold start’ takes still more time for the first position fix as there is no data available to aid signal acquisition and position fix.                After the signal from a satellite has been acquired, the receiver goes to track mode during which it tracks the signal and also downloads the 50 bits/second navigation data. When the signal is strong, the data can be downloaded without error and within the shortest time possible. However, when the received satellite signal is weak due to operation indoors or due to buildings or foliage obstructing the signal, the receiver takes more time to acquire the signal and later during the tracking process it may not be able to correctly down load the associated navigation data. In some cases the correct ephemeris data may be obtained after download of several consecutive frames. This obviously results in a very long wait time. To resolve this problem, the present day receivers receive assistance data containing the current ephemeris through a server or telephone base station. However, this requires additional infrastructure and an arrangement with telephone service providers thus making this process expensive and dependent on many outside factors. There are some patented techniques of downloading the navigation data in a standalone mode. U.S. Pat. No. 5,768,319 assigned to Motorola discloses a method based on overlaying several of the consecutive frames and deciding the value of each bit by computing the average power of the similarly placed bits. Thus this procedure requires downloading a large number of consecutive frames over a long period of time which is unacceptable to the user. U.S. Pat. Nos. 5,731,787 and 5,587,716 disclose a navigation data prediction method that is used when there is no DGPS message reception. A method by Qualcomm is disclosed in the U.S. Pat Pub. No. 2002/0049536, which involves predicting a part of the navigation data (HOW word) by incrementing the Z count of the already received old data. The parity check bits are also changed accordingly. Thus this method is limited to part of the navigation data prediction and under some cases it may obtain assistance data from a server. U.S. Pat. No. 6,515,620 and U.S. Pat. Pub. No. 2005/0035904, which deal with standalone receivers, disclose a technique where only part of the navigation data are predicted.        
Therefore, there is a need for a standalone receiver capable of downloading the entire or most of the navigation data under weak signal conditions at a faster rate.